Based on this theoretical prediction, if dark matter can self-annihilate to give gamma-ray photons, one can expect that the annihilation gamma-ray signals would be greatly enhanced because the annihilation rate is proportional to \(\rho _{\rm DM}^2\) . A lot of attention has been paid specifically for the dark matter density spike surrounding galactic supermassive black holes [1]}, [2]}, [3]}, [4]}, [5]} and intermediate-mass black holes [6]}, [7]}. Various studies have been done to examine the possible enhanced gamma-ray signals, especially near the supermassive black hole in the Milky Way galaxy [3]}, [5]}. However, no promising signals have been observed to verify the theoretical prediction [3]}. However, it does not mean that the dark matter density spike model is wrong. The negative result in gamma-ray observations could be due to the following reasons: 1. the rest mass of dark matter particles is very large, 2. the annihilation cross section is very small, or 3. dark matter particles do not self-annihilate.
